This invention relates to a waveguide-microstripline transformer, which is used in a down converter etc. for broadcasting or communication by man-made satellites, and more particularly to a waveguide-microstripline transformer in which the mode of the electromagnetic wave is transformed from a mode for propagating in a waveguide to a mode for propagating in a microstripline.
In recent years, commercial satellite (CS) broadcasting has become popular, and CS broadcasts which use commercial communication satellites are now being implemented. This has resulted in increased occasions for general households to receive broadcasts from plural satellites. In the course of this development, in addition to the demands for reduced size and lower costs for the receiving antenna, a new problem has arisen due to the interference of a polarized wave from one satellite with a differently polarized wave from another satellite. As such, there is a renewed interest in the importance of a low-noise down-converter having excellent performance, which has the ability to discriminate cross-polarization waves in order to determine when a parabola antenna is used, regardless of whether there is suppression of the interference.
What follows is an explanation of a conventional waveguide-microstripline transformer, as shown in FIGS. 3(a)-3(d). A conventional waveguide-microstripline transformer comprises a cylindrical waveguide 1, a shield case 2, dielectric substrate plate 3, and two microstriplines 4 and 5 working as probes. The shield case 2 or a short cylinder with a bottom plate has an inside diameter the same as the waveguide 1, a depth equal to 1/4 of the wave length and closes the end of the waveguide 1 with a dielectric substrate plate 3 in between. On the dielectric substrate plate 3, there are microstriplines 4 and 5 working as probes.
When an electromagnetic wave (assuming the wave is single polarized) is propagated through the waveguide 1, it is totally reflected by the shield case 2, and the reflected wave excites the microstripline probe 4 so as to be transformed to an electromagnetic wave which propagates along the microstripline. If the incident electromagnetic waves are of cross-polarized type, providing another microstripline probe 5 makes it possible to transform two mutually orthogonal polarized waves into waves propagating on the microstriplines.
However, in the above conventional structure it is necessary to make the waveguide 1 and the dielectric substrate plate 3 perpendicular to each other. This proves problematic when used in combination with a parabolic reflector such as an antenna, because there will be an undesirably large area that can block the electromagnetic wave incident upon the reflector. This conventional structure also is inferior when receiving cross-polarized waves, as the orthogonally polarized waves either interfere with each other or the structure's ability to discriminate between them decreases, because the two microstripline probes 4 and 5 must be formed on the same dielectric substrate plate 3 intersecting the waveguide 1.
There thus exists a need in the art for a waveguide-microstripline transformer that reduces the possibility of the electromagnetic waves being blocked before reaching the reflector. Further, there is a need for a waveguide-microstripline transformer that will effectively separate and discriminate between two incident orthogonally polarized waves.